Osteoclasts, which originate from bone marrow precursor cells, are multinucleated macrophages whose bone absorptive capacity is enhanced in the pathological state of RA and mediated by secretion of hydrochloric acid and associated serine proteases such as cathepsin K and tartrate-resistant acid phosphatase (TRAP) on the surface of bones [3, 4]. Here, ACP5 is linked to rheumatoid arthritis.